This invention relates to a method for making marks under the surface in a body of a glass which has a transmission curve with a plateau area at wavelengths which are greater than those of x-rays, wherein a laser beam is aimed at a surface of the body which can penetrate the body to a predetermined depth of the mark, and also is focused at a predetermined location of the mark within the glass, and which has such a power density that a mark develops at the location in the form of a material alteration distinguished by a lowered permeability to electromagnetic radiation, substantially without any alteration that can be detected in any way at the surface of the body.
In EP 0 543 899 B1 a method is disclosed for producing marks in a glass body in of this type. In this method, laser radiation is used with such an energy density that at the focusxe2x80x94i.e., at the point where the marking is to be donexe2x80x94the energy density suffices to produce permanent alterations within the body, which can consist of glass or even another material. Therein it is described as advantageous if the energy density at the focus of the laser beam amounts to at least 10 J/cm2, since this is approximately the threshold for the occurrence of localized ionization of the glass molecule. According to the known method, laser radiation with a wavelength of 1.06 xcexcm is used for the purpose.
It is a disadvantage that, at this wavelength, which is in the infrared range, the corresponding transmittance for glass is in the plateau part of the transmission curve of the glass. This signifies that, at this wavelength, the transmission of the laser beams through the glass body is approximately at a maximum if the absorption is linear. In order for the desired alteration of the glass to occur in the focus area of the laser beamxe2x80x94that is, the desired nonlinear absorption, a certain energy density threshold must be exceeded, as stated above. If the laser beam is in the infrared range, however, this energy density threshold is very sharp, so that an abrupt transition occurs from linear absorption to the nonlinear absorption producing the mark.
This might be the reason for the fact that, in the known method, an alteration of the glass takes place beyond the actual range of focus of the laser radiation, which is connected with the fact that this bubble-like alteration, which can be explained by a local fusion of the glass, occurs abruptly, almost explosively. This results in the necessity that the mark, which is produced by a series of these point marks, must be spaced a certain minimum distance from the surface of the glass body, since otherwise the points reach from their center beneath the surface of the glass body to the surface and therefore cause a rupture of the glass at the surface. In the known method, the minimum distance of a mark in a glass body from the surface is about 1 millimeter, so that the glass body must have a total thickness of at least 3 millimeters to avoid the risk of breakage.
Furthermore, a method of the kind has been described in xe2x80x9cLaser Magazinxe2x80x9d 1/95, p. 16 ff. This method, in which laser beams were also used with a transmittance within the plateau region of the transmission curve of the glass used, showed the best results in the case of quartz glass with regard to the expansion of the area of fusion, which reached about 100 xcexcm. When different process parameters were used, the expansion of the area of fusion also amounted to several hundred micrometers. This method thus has basically the same disadvantages as the known method described above.
Therefore it is the object of the invention to provide a method of the kind described above in which the individual points which together make the mark can be produced with a very small minimal diameter.
This object is achieved by the method described and claimed herein after. Since a wavelength of the laser light is used at which the glass is partially translucent and which is shorter than all of the wavelengths of the laser light corresponding to the plateau area, the result is that the expansion of the marking points in the glass body can be kept very small. This effect has been found surprising. A possible explanation might be that in wavelengths which correspond to a transmittance below the plateau level, the transition from substantially linear absorption to absorption with a considerable content of nonlinear absorption is xe2x80x9csofter,xe2x80x9d i.e., an energy density range exists in which the proportion of nonlinear absorption gradually increases. For it has been found experimentally that, according to the invention, the expansion of the marking point can be very well controlled by appropriate adjustment of the energy density in the focus area and accordingly even glass bodies of a thickness of only 1 mm can be provided with marking in their interior.
Furthermore, the method of the invention for the interior patterning of glass has the advantage over the state of the art that the laser radiation, due to the shorter wavelengths used, can be focused better and thus additional favorable conditions are created for minimizing the spreading of the focus.
Preferably, a wavelength at which the transmittance is 60 to 95% of the plateau level is selected for the laser radiation.
Also, in the scope of the invention provision is made for the laser radiation to be produced by means of an Nd-YAG laser, using, for example, the third harmonic or also the fourth harmonic.
As a rule the wavelength will be in the UV range. It is important, of course, that the wavelength be made so great that there will be a partial translucency in the glass body, at which sufficient radiation intensity is present at the desired marking location.